On a printed wiring board having high speed integrated circuits thereon, bypass capacitors have been usually placed in the neighborhoods of power supply terminals of each of the integrated circuits.
It has been generally known that a bypass capacitor plays a role to cause a through current generated at a power supply terminal when a high speed integrated circuit is switched to flow back to ground in the neighborhood of the power supply terminal to thereby prevent propagation of high frequency energy (a high frequency component) onto the power supply plane and a role to provide a direct current power supply to the integrated circuit.
That is, if a placement position of a bypass capacitor is inappropriate, high frequency energy from the power supply terminal is not removed sufficiently and spreads all over the power supply plane, leading to a possibility of serving as a source of radiation noise.
A proposal has been offered of placement check of the bypass capacitor, wherein a predetermined value is provided for a distance or the like from the power supply terminal to the bypass capacitor and it is checked whether or not the bypass capacitor has been placed at the predetermined value or less, or a capacity value of the bypass capacitor to be placed is checked.
Besides, it has been checked manually with the naked eye or the like, or alternatively, with a check system whether or not a printed wiring board that has been designed with a printed wiring board CAD does not meet check conditions with respect to placement.
In placement of a bypass capacitor on a printed wiring board, what is important is not only a distance from the power supply terminal to the bypass capacitor and a capacity value thereof, but also a connection sequence of the bypass capacitor and the power supply via hole connected to the power supply plane as viewed from the power supply terminal. In a case where the power supply via hole connected to the power supply plane is nearer the power supply terminal than the bypass capacitor, a through current generated from the power supply terminal flows into the power supply plane before the current flows back to ground the bypass capacitor; therefore, an effect of power supply decoupling is reduced.
In light of the problem, proposals have been offered of a check method or a check system, for example, in Patent document 1 in which it is determined whether or not a decoupling capacitor is inserted in the neighborhood of a power supply terminal of an integrated circuit, where is the optimal insert position and what is the optimal capacity value.